On a bottom layer of a lake or a marsh, a dam, a river, an inner bay, or the like, organic matters resulting from domestic wastewater or agricultural wastewater flowing in from the ground or remains of aquatic plants and planktons that multiply using the organic matters as a nutrition source, are deposited. These organic matters and deposits are decomposed while consuming oxygen contained in bottom layer water. As a result of this decomposition reaction accompanying the oxygen consumption, an oxygen-deficient water area is generated on the bottom layer of the lake or the marsh or the like.
The oxygen-deficient water area refers to an area having a dissolved oxygen concentration as low as 1 to 2 mg/liter, which concentration is far lower than the dissolved oxygen concentration of 10 mg/liter near the surface of the water. The oxygen-deficient water area, in particular, is caught up in a vicious circle. That is, the oxygen-deficient water area is often contaminated, so that photosynthesis cannot take place and algae do not grow, accordingly. Since no algae grow, oxygen is not generated, whereby oxygen deficiency is exacerbated.
It is known that the oxygen deficiency of the bottom layer has various adverse effects on environments of lakes and marshes and the like. For example, if the bottom layer is in an oxygen-deficient state, benthoses often become extinct. If the bottom layer becomes oxygen-deficient, then a reducing atmosphere is established, and metals are eluted from surrounding rocks and stones and from bottom sludge, often resulting in water pollution.
To eliminate such an oxygen-deficient state, there are conventionally known methods for supplying oxygen to the oxygen-deficient water area and increasing the dissolved oxygen concentration. A method for directly supplying bubbled oxygen or the air to the oxygen-deficient water area is disclosed in, for example, Japanese Patent Application Laid-Open No. H5-168981 entitled “oxygen blowing apparatus”, Japanese Patent Application Laid-Open No. H7-185281 entitled “gas dissolving apparatus”, and Japanese Patent Application Laid-Open No. 2002-200415 entitled “apparatus for dissolving air into water”.
A method for forcedly dissolving oxygen into water by pressurizing and mixing up the oxygen and the water in a sealed tank, producing water having an increased dissolved oxygen concentration (hereinafter, referred to as “high dissolved oxygen concentration water” as appropriate), and supplying the high dissolved oxygen concentration water to the oxygen-deficient water area is disclosed in Japanese Patent Application Laid-Open No. 2002-177953 entitled “an automatic dissolved oxygen control method for underwater installation type pressurized tank water”, and Japanese Patent Application Laid-Open No. 2000-245295 entitled “apparatus for supplying oxygen-rich water”.
A method for generating high dissolved oxygen concentration water in a similarly sealed tank, temporarily releasing the generated water into the air in the tank, and supplying the high dissolved oxygen concentration water to the oxygen-deficient water area is disclosed in Japanese Patent Application Laid-Open No. H11-207162 entitled “pressurization type oxygen dissolving method”. A method for filling a to-be-dissolved gas into a sealed tank, ejecting water into the tank, and dissolving the gas into the water is disclosed in Japanese Patent Application Laid-Open No. 2002-346351 entitled “gas dissolving apparatus”.
Patent Document 1: Japanese Patent Application Laid-Open No. H5-168981 Patent Document 2: Japanese Patent Application Laid-Open No. H7-185281 Patent Document 3: Japanese Patent Application Laid-Open No. 2002-200415 Patent Document 4: Japanese Patent Application Laid-Open No. 2002-177953 Patent Document 5: Japanese Patent Application Laid-Open No. 2000-245295 Patent Document 6: Japanese Patent Application Laid-Open No. H1-207162 Patent Document 7: Japanese Patent Application Laid-Open No. 2002-346351